A helmet comprising an impact mitigating structure

ABSTRACT

The present invention relates to a helmet comprising an impact mitigating structure, the impact mitigating structure comprising: a first layer; and a second layer; wherein one or more of a material property, a mechanical property and a geometrical property of the impact mitigating structure is arranged to, when the impact mitigating structure is subject to an impact, facilitate at least partial fracturing of the second layer such that at least a portion of the second layer is able to move relative to the first layer.

This invention relates to an impact mitigating structure, in particular an impact mitigating structure having multiple layers.

Injury to a person or damage to an object can occur when the person or object is subjected to an impact of sufficient magnitude. Considerable developmental effort has been expended to produce impact mitigating structures, and in particular helmets and body armour, which provide protection from potentially damaging or injurious impacts.

Head injuries, which can be incurred as a result of participation in sports such as cycling, horse riding or rock climbing, are a common cause of serious brain injuries. A brain trauma may occur as a consequence of either a focal impact upon the head, a sudden acceleration or deceleration within the cranium, or a combination of both impact and movement. Impact protection is therefore important in preventing brain injuries as a result of impacts to the head.

Head protection, in the form of helmets, is designed to reduce the forces experienced by a user's head during an impact. Generally, a helmet comprises at least one impact absorbing layer which is designed to absorb a portion of the forces to which the helmet is subjected during an impact. Body armour similarly protects other parts of a body.

However, helmets and body armour often do not provide adequate protection during an impact against both linear and tangential forces. As oblique impacts are common, impacts will often include both linear and tangential components. Tangential forces in particular result in the rotational acceleration of the brain, which has been linked to bridging vein rupture. In turn, this may be responsible for subdural haematomas, and diffuse axonal injuries. Tangential forces during an impact may also result in neck injuries.

It is an aim of the present invention to provide improved impact mitigating structures.

When viewed from a first aspect the invention provides a helmet comprising an impact mitigating structure, the impact mitigating structure comprising:

-   -   a first layer; and     -   a second layer;     -   wherein one or more of a material property, a mechanical         property and a geometrical property of the impact mitigating         structure is arranged to facilitate, when the impact mitigating         structure is subject to an impact, at least partial fracturing         of the second layer such that at least a portion of the second         layer is able to move relative to the first layer.

Thus the invention provides a helmet including an impact mitigating structure. The impact mitigating structure is formed from at least two layers.

When the helmet, in particular the impact mitigating structure of the helmet, is subject to an impact, a portion of the force from the impact is transferred to the second layer. This may cause the second layer to fracture (e.g. break) and thus for there to be formed at least two distinct (e.g. completely separated or only partially connected) portions of the second layer. At least one of these portions (of the second layer) is then able to move with respect to the first layer. The movement of the (first and second) layers relative to each other may occur during and after an impact.

The skilled person will appreciate that providing a second layer that is arranged to fracture, which facilities movement of (at least) a portion of the second layer with respect to the first layer, helps some of the energy from the impact to be transferred into the movement of the layers of the helmet with respect to each other. This helps to absorb, deflect and/or dissipate energy from an impact, such that the energy transferred through to the head protected by the helmet is reduced. This helps to reduce the likelihood of head and brain injuries.

Thus the (e.g. fractured) portion of the second layer that moves with respect to the first layer helps to convert energy from the impact into linear and/or rotational movement of the layers relative to each other. The (e.g. fractured) portion of the second layer also reduces the effective size of the second layer (e.g. relative to the rest of the impact mitigating structure), thus helping to reduce the likelihood of geometrical locking of the second layer on the rest of the impact mitigating structure, which also facilitates movement of the layers relative to each other. This helps to reduce the linear and rotational forces transmitted through the helmet to the head protected by the helmet, therefore reducing the force exerted on the head and the rotational movement of head during and after an impact. As will be appreciated, this can reduce the likelihood of head and brain injuries.

The (at least partial) fracturing of the second layer may comprise any suitable and desired type of fracturing. For example, the second layer may be arranged to (at least partially) fracture in the normal direction and/or tangential direction (relative to the surface of the second layer). Tangential fracturing may comprise dislocation (such as exhibited by metal lattice materials) and/or slip bands (such as exhibited by polymer materials).

The helmet may be arranged such that any part of the impact mitigating structure may be subject to an impact that causes fracturing of the second layer, for example depending relative configuration of the (first and second) layers of the impact mitigating structure. Thus the first layer and the second layer may be arranged in any suitable and desired manner relative to each other. Preferably the impact mitigating structure is arranged such that the second layer is subject to the impact which facilitates at least partial fracturing of the second layer. In a set of embodiments, the first layer is an inner layer and the second layer is an outer layer. For example, the first layer is closer to a user's head and the second outer layer is further away from the user's head.

The first layer and the second layer may have any suitable (e.g. overall) geometries, both individually and collectively. Preferably the first layer and/or the second layer has a thickness that is less than their other two dimensions (e.g. across the surface area of the respective layer). In a set of embodiments, the first layer has a thickness that is greater than the thickness of the second layer. Preferably the thickness of one or more (e.g. each) of the first layer and the second layer is substantially constant (e.g. across the surface area of the respective layer).

In a preferred embodiment, the first layer and the second layer are arranged substantially parallel to each other. Thus preferably the first layer and the second layer are stacked on top of each other, e.g. with the first layer being arranged to be positioned, in use, closer to the user's head than the second layer.

In a preferred embodiment, the first layer and/or the second layer is curved (e.g. over the surface area of the respective layer). Preferably the shape and curvature of the first layer and the second layer is such that the layers conform to each other (e.g. the outer surface of the inner (e.g. first) layer and the inner surface of the outer (e.g. second) layer). Preferably the inner (e.g. first) layer has a convex surface facing the outer (e.g. second) layer (and preferably a concave surface on its opposite surface facing away from the outer (e.g. second) layer, e.g. facing a user's head). Preferably the outer (e.g. second) layer has a concave surface facing the inner (e.g. first) layer (and preferably a convex surface on its opposite surface facing away from the inner (e.g. first) layer, e.g. facing away from the user's head).

In preferred embodiments, the (e.g. second layer of the) impact mitigating structure is arranged to introduce (or set) a particular (e.g. threshold) force (to which the impact mitigating structure is subject in an impact) at (or above) which the second layer is arranged to (at least partially) fracture. Thus, when the force of the impact is at least the particular (e.g. threshold) force, the second layer is arranged to (at least partially) fracture.

This may provide a helmet that is particularly suited to its use, as the fracture threshold may be set to match the energy of a typical impact. For example, by setting a high threshold at which the second layer (starts to) fractures, the amount of energy transferred to the layers (e.g. the second layer) to facilitate movement may be increased. This may reduce the energy transfer to the head protected by the helmet in an impact, therefore reducing the likelihood of head or brain injuries.

However, the Applicant has appreciated that there are embodiments in which it is desirable to have a low threshold at which the second layer fractures. By having a low threshold at which the second layer fractures, the movement of the layers with respect to each other may still be facilitated in lower energy impacts, thus helping to reduce the energy transferred to the head protected by the helmet, helping to reduce the likelihood of injuries.

The particular (threshold) force required for the second layer to fracture may be chosen to have any suitable and desired value. In one embodiment, the particular force is between 10 N and 100 N, e.g. between 30 N and 70 N, e.g. approximately N. The particular force may be chosen such that it reflects the lowest range of forces acting on the helmet which may cause damage (e.g. injury) to the head that the helmet is protecting, or such that it reflects the maximum force the user protected by the helmet (or the environment in which the helmet is being used) can exert on the impact mitigating structure (e.g. during normal use, other than undergoing an impact).

The invention provides an impact mitigating structure in which the second (e.g. outer) layer is arranged to (at least partially) fracture as a result of an impact. The second layer may be arranged to fracture in any suitable and desired way, i.e. owing to one or more of a material property, a mechanical property and a geometrical property of the impact mitigating structure.

Thus, in one embodiment a material property of the (e.g. first layer and/or the second layer of the) impact mitigating structure is arranged to facilitate at least partial fracturing of the second layer such that at least a portion of the second layer is able to move relative to the first layer, when the impact mitigating structure is subject to an impact.

In such embodiments, the material properties of the impact mitigating structure may be specifically set during manufacture to facilitate controlled and predictable fracturing of the second layer. The material property (or properties) of the impact mitigating structure may comprise one or more of a material property of the first layer, a material property of the second layer, or a material property of any intermediate layer or connection between the first and second layers, for example.

The material property may comprise, for example, a bulk material property of (a component of) the impact mitigating structure. In one embodiment the second layer has a fracture toughness of between 0.1 MPa m^(1/2) and 10 MPa m^(1/2), e.g. between MPa m^(1/2) and 5 MPa m^(1/2), e.g. between 1 MPa m^(1/2) and 3 MPa m^(1/2). The fracture toughness may therefore be chosen to set a particular threshold force at which the second layer is arranged to (start to) fracture.

Other material properties, such as the bulk material composition, the porosity of the material and/or the fracture strength of the material (e.g. of the second layer) may be selected to control the susceptibility of the second layer to fracture, when subject to an impact.

The (bulk) material of the second layer may be chosen to facilitate fracturing of the second layer, e.g. when the impact mitigating structure is subject to an impact at or above the particular (threshold) force. Possible materials for the impact mitigating structure are outlined below.

In one embodiment a mechanical property of the (e.g. first layer and/or the second layer of the) impact mitigating structure is arranged to facilitate at least partial fracturing of the second layer such that at least a portion of the second layer is able to move relative to the first layer, when the impact mitigating structure is subject to an impact.

The mechanical property of the impact mitigating structure may comprise any suitable and desired mechanical property of the first layer, the second layer or other part of the impact mitigating structure, e.g. how these components interact mechanically with each other when the impact mitigating structure is subject to an impact. In one embodiment the first layer and/or the second layer comprises one or more protrusions (e.g. raised point(s) or line(s)) arranged to facilitate (e.g. initiate) at least partial fracturing of the second layer when the impact mitigating structure is subject to an impact.

Preferably the protrusion(s) on the first layer and/or the second layer are on the surface of the first layer and/or the second layer that faces the respective other layer, e.g. a protrusion on the first layer is on the surface facing the second layer and vice versa. It will be appreciated that such protrusion(s) may help to concentrate stress on the second layer from the force of the impact, helping to facilitate the fracturing of the second layer.

In one embodiment the impact mitigating structure comprises one or more fracture initiating members adjacent the second layer, wherein the one or more fracture initiating members are arranged to facilitate (e.g. initiate) at least partial fracturing of the second layer when the impact mitigating structure is subject to an impact. Preferably the fracture initiating member(s) are in contact with the second layer or is arranged to contact the second layer when the impact mitigating structure is subject to an impact. Thus preferably the fracture initiating member(s) are arranged to exert a force against the second layer when the impact mitigating structure is subject to an impact. The fracture initiating member(s) thus act to concentrate stress on the second layer, from the force of an impact, facilitating fracturing of the second layer.

The fracture initiating member(s) may be located in any suitable and desired position in the impact mitigating structure, so to act against the second layer to facilitate fracturing of the second layer. In one embodiment the fracture initiating member(s) are between another (e.g. the first) layer of the impact mitigating structure and the second layer, e.g. the fracture initiating member(s) may be provided as part of an additional (intermediate) layer.

The fracture initiating member(s) may take any suitable and desired form. In one set of embodiments the fracture initiating member(s) comprise one or more raised points or lines (e.g. balls or strips).

In one embodiment a geometrical property of the (e.g. first layer and/or the second layer of the) impact mitigating structure is arranged to facilitate at least partial fracturing of the second layer such that at least a portion of the second layer is able to move relative to the first layer, when the impact mitigating structure is subject to an impact. Thus preferably the (e.g. first layer and/or the second layer of the) impact mitigating structure is shaped to facilitate at least partial fracturing of the second layer.

Preferably the second layer is shaped to form one or more points of weakness in the second layer, wherein the one or more points of weakness are arranged to facilitate at least partial fracturing of the second layer. It will be appreciated, as will be discussed below, that one or more points of weakness of may stem from a material property (e.g. non-uniformity of the material) of the impact mitigating structure, as well as or instead of from a geometrical property.

Preferably the second layer comprises one or more (e.g. a plurality of) points of weakness that are arranged to facilitate, when the (e.g. second layer of the) impact mitigating structure is subject to an impact, at least partial fracturing of the second layer, such that at least a portion of the second layer is able to move relative to the first layer. Thus, in these embodiments, the second layer has one or more (e.g. predefined) points of weakness, e.g. point(s) of the second layer that are formed as being (intentionally) weaker than surrounding parts of the second layer.

The point(s) of weakness (and/or the protrusion(s) and/or the fracture initiating member(s) outlined above) are arranged such that they facilitate (at least partial) fracturing of the second layer in an impact, which facilities the movement of the first layer and the second layer with respect to each other. This may cause the second layer to fracture (e.g. break), at (or extending between) one or more of the point(s) of weakness (and/or the location of the second layer at the protrusion(s) and/or the fracture initiating member(s)), and thus for there to be formed at least two distinct (e.g. completely separated or partially connected) portions of the second layer. For example, the point(s) of weakness (and/or the protrusion(s) and/or the fracture initiating member(s)) may act to initiate the (at least partial) fracturing of the second layer, as a result of an impact.

Preferably, the second layer is arranged to at least partially fracture at at least one of the point(s) of weakness, when the second layer is (and/or the location of the second layer at the protrusion(s) and/or the fracture initiating member(s)), when the impact mitigating structure is subject to an impact, e.g. having at least a particular (e.g. predefined, threshold) force. Preferably, the point(s) of weakness are (e.g. particular, predefined) parts of the second (e.g. outer) layer which are weaker than other parts (e.g. the rest) of the second layer. Thus the protrusion(s), the fracture initiating member(s) and/or the point(s) of weakness (e.g. their number, shape, size and/or distribution) may be used to set the particular (e.g. predefined, threshold) force.

For example, the energy (and thus the force of the impact) required to fracture the second layer at a point of weakness (and/or the location of the second layer at the protrusion(s) and/or the fracture initiating member(s)) is (e.g. significantly) less than the energy which would be required to fracture another part (e.g. the rest) of the second layer. Typically, the energy required to fracture the second layer at a point of weakness (and/or the location of the second layer at the protrusion(s) and/or the fracture initiating member(s)) is (e.g. significantly) less than the energy that would be required to fracture any point on a homogenous second (e.g. outer) layer (e.g. having similar properties to the second layer away from the point(s) of weakness (and/or away from the location of the second layer at the protrusion(s) and/or the fracture initiating member(s))). This encourages fracturing to occur at one or more of the point(s) of weakness (and/or the location of the second layer at the protrusion(s) and/or the fracture initiating member(s)), as opposed to at another part of the second layer.

The point(s) of weakness (and/or protrusion(s) and/or fracture initiating member(s)) may be arranged to facilitate the (at least partial) fracturing of the second layer in any suitable and desired manner. In preferred embodiments, the second layer is arranged to fracture at at least one of the point(s) of weakness (and/or the location of the second layer at the protrusion(s) and/or the fracture initiating member(s)). For example, the second layer may be arranged to fracture along a line that passes through at least one of the point(s) of weakness (and/or the location of the second layer at the protrusion(s) and/or the fracture initiating member(s)). In some (potentially overlapping) embodiments, the second layer is arranged to fracture (along a line that extends) between at least two points of weakness (and/or two locations of the second layer at protrusions and/or fracture initiating members) (e.g. with the points of weakness (and/or the locations of the second layer at the protrusions and/or the fracture initiating members) at the ends of the line). For example, the (e.g. material of the) second layer and/or the points of weakness may be arranged such that fracture(s) may propagate between various of these points of weakness (and/or locations of the second layer at protrusions and/or fracture initiating members).

The (plurality of) protrusions, fracture initiating members and/or points of weakness may be arranged (e.g. geometrically) in any suitable and desired manner. In some embodiments, the (plurality of) points of protrusions, fracture initiating members and/or weakness are distributed randomly over the second layer. Preferably, the (e.g. plurality of) protrusions, fracture initiating members and/or point(s) of weakness are arranged at particular (e.g. predefined) location(s) on the second layer, e.g. in a regular array or relative to other feature(s) of the second layer (such as vent(s) of the helmet).

In a set of embodiments, the (plurality of) points of weakness are arranged (e.g. continuously, in one or more lines) to form one or more lines of weakness. Similarly, a plurality of discrete protrusions and/or fracture initiating members may be arranged (e.g. continuously, in one or more lines) to form one or more lines of protrusions and/or fracture initiating members. For example, the (plurality of) points of weakness (and/or protrusions and/or fracture initiating members) may be arranged in a particular path (or sequence) across the second layer. The plurality of points of weakness may therefore comprise one or more perforations in the second (e.g. outer) layer. This may encourage the propagation of fractures along the line(s) of weakness (and/or protrusions and/or fracture initiating members) when the impact mitigating structure is subject to an impact having at least a particular (e.g. predefined, threshold) force, such that the fracturing of the second layer occurs in a particular (e.g. predefined) and controlled manner.

In a set of embodiments, the point(s) of weakness comprise one or more lines of weakness (and thus preferably the second layer comprises one or more lines of weakness arranged to facilitate, when the impact mitigating structure is subject to an impact, at least partial fracturing of the second layer). For example, the point(s) of weakness may comprise groove(s) (e.g. narrow channel(s)) in the second layer. Similarly, as outlined above, the protrusion(s) and/or fracture initiating member(s) may comprise raised line(s).

Preferably, when the (e.g. second layer of the impact mitigating structure) is subject to an impact having at least a particular (e.g. predefined, threshold) force, the second layer is arranged to (at least partially) fracture along at least one of the one or more lines of weakness (and/or line(s) of protrusion(s) and/or fracture initiating member(s)). This encourages predictable and controlled fracturing of the second layer as a result of an impact.

The one or more lines of weakness (and/or line(s) of protrusion(s) and/or fracture initiating member(s)) may be arranged in any suitable and desired manner. In some embodiments, the line(s) of weakness (and/or line(s) of protrusion(s) and/or fracture initiating member(s)) may be distributed randomly. Preferably, the one or more lines of weakness (and/or line(s) of protrusion(s) and/or fracture initiating member(s)) are arranged at particular (e.g. predefined) location(s) on the second layer, e.g. in a regular (geometrical) pattern or relative to other feature(s) of the second layer (such as vent(s) of the helmet).

In one set of embodiments the plurality of points (e.g. the one or more lines) of weakness (and/or protrusions and/or fracture initiating members) are arranged to define one or more segments of the second layer. This may encourage one or more of these segment(s) of the second layer to be the portions of the second layer that move relative to the first layer when the helmet is subject to an impact. It will be appreciated that when a segment of the second layer fractures from the rest of the second layer, it helps to reduce the effective surface area of the second layer (e.g. relative to the rest of the impact mitigating structure), thus helping to reduce the likelihood of geometrical locking of the second layer on the rest of the impact mitigating structure, which facilitates movement of the layers relative to each other.

Thus, in some embodiments, two or more lines (e.g. of a plurality of points) of weakness (and/or line(s) of protrusion(s) and/or fracture initiating member(s)) are arranged to intersect. In some embodiments, the lines of weakness do not intersect each other but may still, for example, define one or more segments on the second layer.

The plurality of points or lines of weakness (and/or protrusion(s) and/or fracture initiating member(s)) may be distributed evenly (e.g. having equal separations) across the second layer, e.g. in a regular array or (geometrical) pattern. In some embodiments, the separation of the points or lines of weakness (and/or protrusion(s) and/or fracture initiating member(s)) may depend on their position (location) on the second layer. For example, the lines of weakness (and/or protrusion(s) and/or fracture initiating member(s)) may be more closely spaced (e.g. there may be smaller separations between the lines of weakness (and/or protrusion(s) and/or fracture initiating member(s))) towards the edges of the second layer and the lines of weakness (and/or protrusion(s) and/or fracture initiating member(s)) may be more sparsely spaced (e.g. there may be larger separations between the lines of weakness (and/or protrusion(s) and/or fracture initiating member(s))) towards the geometrical centre of the second layer.

In a set of embodiment, the point(s) and/or line(s) of weakness (and/or protrusion(s) and/or fracture initiating member(s)) are arranged to facilitate (at least partial) fracturing of the second layer in a particular (e.g. predefined) direction on the second layer. For example, by arranging point(s) and/or line(s) of weakness (and/or protrusion(s) and/or fracture initiating member(s)) in a particular direction on (or with respect to) the second layer, fracturing (as a result of an impact) is encouraged in the direction of the point(s) and/or line(s) of weakness. In one embodiment, progressively weaker points and/or lines (and/or progressively larger protrusions and/or fracture initiating members) may be arranged to define a preferential direction for fracturing, e.g. to facilitate propagation of fractures.

The arrangement of the point(s) and/or line(s) of weakness (and/or protrusion(s) and/or fracture initiating member(s)) may reduce the force required to fracture the second layer in the particular direction (e.g. compared with another direction), therefore biasing a fracture to occur upon impact in this direction. Encouraging fracturing upon impact in the direction of structural (or other) feature(s) in the first and/or second layers may reduce the risk of the structural (or other) feature(s) obstructing the movement of the layers with respect to each other. For example, this may help to reduce the risk of geometrical locking of the second layer in vents of the helmet.

In a set of embodiments, the (plurality of) points and/or lines of weakness (and/or protrusion(s) and/or fracture initiating member(s)) may vary (e.g. in form, size, shape) depending on their location (in, on or relative to the second layer). It may be desirable for certain region(s) of the helmet to fragment and move relative to the first (e.g. inner) layer in an impact. These regions may correspond to sections of the helmet where geometric locking may occur, e.g. where structural (or other) feature(s) of the first layer and/or second layer interact to obstruct and/or prevent the movement of the layers with respect to each other.

For example, the curvature of the first layer and the second layer, and/or protruding or recessed features (such as vents and/or attachment points), may prevent (at least a portion of) the first layer and (at least a portion of) the second layer from translating (e.g. smoothly) with respect to each other. An increased number, larger, and/or more easily fractured points or lines of weakness (and/or protrusions and/or fracture initiating members) may be provided in such region(s), to assist with fracturing and movement of the layers in these region(s) as a result of an impact.

The point(s) and/or line(s) of weakness may have any suitable and desired form (e.g. structure), and may be formed (e.g. manufactured) in any suitable and desired manner.

In a set of embodiments, the point(s) and/or line(s) of weakness are defined or formed by one or more material properties of the second layer. The one or more material properties of the second layer may be formed (during manufacture) at predefined point(s) and/or line(s) in the second layer, such that these predefined point(s) and/or line(s) are weaker than other parts of the second layer. In such embodiments, the one or more material properties of the second layer are specifically set during manufacture to facilitate controlled and predictable fracturing of the second layer at (or between) the point(s) and/or along line(s) of weakness.

The second layer may have any suitable and desired one or more material properties to provide the point(s) and/or line(s) of weakness. In a set of embodiments, the second layer comprises (is formed of) a material having one or more impurities therein, wherein the one or more impurities define the point(s) and/or line(s) of weakness. For example, the impurities may be arranged to reduce the toughness and/or strength of the second layer by disrupting the arrangement of an otherwise homogenous material.

Example of impurities include fibres and seeding particles (e.g. dust) in the material of the second layer. In one embodiment, e.g. when the second layer is formed by injection moulding, fibre(s) may be introduced during the (e.g. injection moulding) manufacturing process of the second layer to form line(s) of weakness (i.e. along the fibres). In one embodiment, seeding particles (such as dust) may be introduced during formation of the second layer to facilitate the formation of one or more grain boundaries (thus providing the line(s) of weakness). Grain boundaries may introduce variable stresses across the second layer, forming specific points and/or lines which are more susceptible (than the bulk material) to fracturing as a result of an impact.

The concentration of the impurities, as well as other variables such as the size and/or shape of any fibres or seeding particles in the second layer, may be selected to control the distribution and/or (relative) strength of the point(s) and/or line(s) of weakness. Other material properties, such as the bulk material composition and/or the porosity and/or the fracture toughness of the material and/or the fracture strength of the material of the second layer may be selected to control the distribution of the points and/or lines of weakness, and/or the susceptibility of the second layer to fracture. For example, the second layer at the points and/or lines of weakness may has a fracture toughness of between 0.1 MPa m^(1/2) and 10 MPa m^(1/2), e.g. between 0.5 MPa m^(1/2) and 5 MPa m^(1/2), e.g. between 1 MPa m^(1/2) and 3 MPa m^(1/2).

In a set of embodiments, the point(s) and/or line(s) of weakness are defined or formed by one or more geometrical properties of the second layer. For example, the point(s) of weakness and/or line(s) of weakness may comprise perturbation(s) in the second layer, e.g. variation(s) from a (e.g. uniform) thickness of the second layer. For example, the thickness of the (material of the) second layer at the point(s) and/or lines of weakness may be less than the surrounding region(s) (material) of the second layer.

In a set of embodiments, the point(s) and/or line(s) of weakness comprise indentation(s) (e.g. notch(es)) or void(s) in the second layer. Preferably the thickness of the second layer at an indentation is less than the thickness of the second layer surrounding the indentation. Preferably a void in the second layer (e.g.

a hollow portion of the second layer) has less material than the second layer surrounding the void.

The indentation(s) (e.g. notch(es)) or void(s) may be any suitable and desired shape and/or size. The indentation(s) (e.g. notch(es)) may be located on the outer surface of the second layer (the surface facing away from the head) and/or on the inner surface of the second layer (the surface facing toward the head). In embodiments in which indentations (e.g. notches) are arranged on both the outer and inner surface of the second layer, the indentation(s) (e.g. notch(es)) on the outer surface may be adjacent to the indentation(s) (e.g. notch(es)) on the inner surface. For example, an indentation in the outer surface may correspond to an indentation at the same location on the inner surface.

In a set of embodiments, the point(s) and/or line(s) of weakness comprise aperture(s) extending through the (thickness of the) second layer (as opposed to indentation(s) that extend only partially through the second layer). The aperture(s) may be any suitable and desired shape and/or size. The aperture(s) may have any suitable and desired width (i.e. the dimension parallel to the surface of the second layer). Preferably, the width (e.g. diameter) of the aperture(s) is substantially equal to the thickness of the second layer.

The indentation(s) (e.g. notch(es)) and/or void(s) and/or aperture(s) may be arranged in any suitable and desired configuration on or in the second layer. In a set of embodiments, the (plurality of) points and/or line(s) of weakness comprise one or more sets of perforations (in the second layer). In some embodiments, the perforations are formed from a plurality of apertures (as described above) arranged in one or more lines. Similarly, a plurality of indentations (e.g. notches) or voids may be arranged in line across the surface(s) of the second layer. This may facilitate the second layer to (at least partially) fracture along a perforation or a line of indentations (e.g. notches) or voids.

In a set of embodiments, the point(s) and/or line(s) of weakness comprise groove(s) on the surface of the second layer or slot(s) extending through the second layer or longitudinally extended voids within the second layer. For example, groove(s) or slot(s) or void(s) may provide (e.g. thin) line(s) of weakness across the surface of the second layer. The groove(s) or slot(s) may thus be seen as indentation(s) in or aperture(s) through or void(s) within the second layer that extend in line(s) over the surface of, or within, the second layer.

The groove(s) or slot(s) or void(s) may have any suitable and desired dimensions. Preferably, the length of a groove or slot or void (parallel to the surface of the second layer) is (substantially) greater than the width of the groove or slot or void (perpendicular to the length and parallel to the surface of the second layer). Preferably, the length of a groove or slot or void (parallel to the surface of the second layer) is (substantially) greater than the thickness of the second layer).

Groove(s) may be located on the outer surface of the second layer (the surface facing away from the head) and/or on the inner surface of the second layer (the surface facing toward the head). In embodiments in which groove(s) are arranged on both the outer and inner surface of the second layer, the groove(s) on the outer surface may be adjacent to the groove(s) on the inner surface. For example, a groove in the outer surface may correspond to a groove at the same location on the inner surface. Groove(s) may help to facilitate fracturing of the second layer as a result of an impact by providing a starting point for the propagation of fracture(s).

When there are a plurality of points and/or lines of weakness (and/or protrusions and/or fracture initiating members), the points and/or lines of weakness (and/or protrusions and/or fracture initiating members) may be identical (e.g. in terms of their form, shape and size) throughout the second layer. This may help to facilitate consistent fracturing of the second layer. However, in some embodiments the (plurality of) points and/or lines of weakness (and/or protrusions and/or fracture initiating members) vary (e.g. in their form, size and/or shape) throughout the second layer.

The (plurality of) points and/or lines of weakness (and/or protrusions and/or fracture initiating members) may comprise one or more of the (e.g. types of) points and/or lines of weakness (and/or protrusions and/or fracture initiating members) described herein. Variations in the (plurality of) points and/or lines of weakness (and/or protrusions and/or fracture initiating members) may help to optimise the fracturing of the second layer, e.g. depending on the geometry of the (e.g. layers of the) helmet and/or the likely impacts that the helmet may experience.

As outlined above, preferably, the (plurality of) points and/or lines of weakness (and/or protrusions and/or fracture initiating members) are arranged to define (the boundary or boundaries of) one or more segments (regions) of the second layer. In such embodiments, the second layer comprises one or more segments, e.g. that form the one or more portions of the second layer which are able to move relative to the first layer when the second layer (at least partially) fractures when the impact mitigating structure is subject to an impact.

The points and/or lines of weakness (and/or protrusions and/or fracture initiating members) may define any suitable and desired number of segments of the second layer. Preferably, the (plurality of) points and/or lines of weakness (and/or protrusions and/or fracture initiating members) define a plurality segments (regions) of the second layer. Preferably, the second layer comprises between 3 and 1000 segments, e.g. between 50 and 500 segments, e.g. between 75 and 300 segments, e.g. between 100 and 150 segments. The number of segments in the second layer may depend on the shape of the helmet and/or the likelihood of geometric locking as a result of an impact. For example, the number of segments may be related to the number of vents of the helmet. In some embodiments, the plurality of segments may extend over the entirety of the second layer.

The one or more segments may be positioned (or arranged relative to each other) in any suitable and desired way. For example, the segment(s) may be arranged randomly on the second layer or the segments may form a regular array. In one embodiment the segment(s) are arranged relative to (e.g. surrounding) the geometrical feature(s) of the (e.g. first and/or second layers of the) helmet. In one embodiment the segment(s) are arranged to surround one or more vents in the (e.g. first and/or second layers of the) helmet.

Preferably, when the (e.g. second layer of the) impact mitigating structure is subject to an impact (e.g. having at least a particular (e.g. predefined, threshold) force), the second layer (e.g. at at least one point and/or line of weakness (and/or the location of the second layer at the protrusion(s) and/or the fracture initiating member(s))) is arranged to fracture to facilitate the (at least partial) detachment of at least one portion (e.g. segment) from the (e.g. remaining portion of the) second layer. The (at least partial) detachment of the at least one portion (e.g. segment) from the second layer is arranged to facilitate the movement of (e.g. the (at least partially) detached portion and/or the remaining portion of) the second layer with respect to the first layer. Thus, the first layer and (e.g. at least a portion of) the second layer may be arranged to (e.g. completely) separate from each other when the impact mitigating structure is subject to an impact (e.g. having a force that is greater than equal to the particular force), e.g. to facilitate movement of the layers relative to each other.

Thus, the portion (e.g. segment) of the second layer that is able to move relative to the first layer may comprise one or both of the detached portion and the remaining portion of the second layer. In one embodiment, the second layer is arranged to fracture such that detachment of at least one portion of the second layer releases the second layer (e.g. from the first layer) and allows the (e.g. remaining portion of the) second layer to move relative to the first layer (e.g. as well as the detached portion of the second layer).

In a set of embodiments, when the impact mitigating structure is subject to an impact having at least a particular (e.g. predefined, threshold) force, the fracturing of the second layer is arranged to fragment the second layer into a plurality of (e.g. detached, separate) portions, e.g. segments defined by the points and/or lines of weakness (and/or protrusions and/or fracture initiating members). Fragmenting at least some of the second layer into a plurality of portions helps the portions move independently of each other.

This is because the fragmenting of the second layer may help to reduce the likelihood of the movement of the (e.g. portion(s) of the) second layer relative to the first layer being obstructed and/or prevented (e.g. by geometrical locking of the second layer on structural features in the first layer), owing to the smaller size of the fragmented portions of the second layer. For example, vents (e.g. on a bicycle helmet) and/or notches (e.g. visor mounts on a motorcycle helmet) in the first layer may otherwise obstruct and/or prevent the movement of the second layer.

In some embodiments, the (at least partially) detached portion(s) (e.g. fragment(s) and/or segment(s)) facilitate the movement of the second layer with respect to an impacting object, as well as relative to the first layer. For example, the detached portion(s) may be arranged to provide a low friction, translating (e.g. rolling) surface against the impacting object, which may, for example, comprise a solid, static object such as the ground. This may help to facilitate the movement of the first layer with respect to the impacting object, and reduce the energy transferred to the head protected by the helmet.

The (e.g. second layer of the) impact mitigating structure may be arranged to fracture such that the detached portion(s) (e.g. fragment(s) and/or segment(s)) are able (e.g. free) to move relative to the first layer, when the second layer fractures. In a set of embodiments, the detached portion(s) are arranged to be freed (and, e.g., ejected) from the impact mitigating structure, thus allowing them to move relative to the first layer. This may (also) facilitate the movement of a remaining portion of the second layer with respect to the first layer, e.g. by reducing geometrical locking and/or by releasing the remaining portion of the second layer from the first layer (such as when the detached portion was connecting the second layer to the first layer). The ejection of the detached portion(s) may also dissipate the energy from an impact, such that the energy transferred through to the head protected by the helmet is reduced.

The (e.g. second layer of the) impact mitigating structure may be arranged to fracture such that at least one portion of (and that has fractured from) the second layer remains partially attached to the second layer. For example, the point(s) and/or line(s) of weakness (and/or protrusion(s) and/or fracture initiating member(s)) may be arranged such that they do not define discrete segment(s) or only a portion of the point(s) and/or line(s) of weakness (and/or protrusion(s) and/or fracture initiating member(s)) defining a segment may be arranged to fracture when the (e.g. second layer of the) impact mitigating structure is subject to an impact.

However, the remaining point(s) and/or line(s) of weakness (and/or the second layer at the protrusion(s) and/or fracture initiating member(s)) defining a segment may be weakened (e.g. partially fractured) as a result of the impact (with the second layer fracturing around the rest of the perimeter of the segment). This may facilitate movement (e.g. flexing) of the partially attached segment with respect to (e.g. remaining portion of) the second layer. For example, the second layer may be arranged to bend (e.g. along the weakened point(s) or line(s) and/or the second layer at the protrusion(s) and/or fracture initiating member(s)) between the partially detached segment and the (e.g. remaining portion of) the second layer. This may enable deformation of the second layer (thus, for example, releasing a portion of the second layer from (e.g. connection with) the first layer), which may help to facilitate movement of the first layer and the second layer with respect to each other.

In some embodiments, the second layer is arranged to (at least partially) fracture (e.g. at the point(s) and/or line(s) of weakness (and/or protrusion(s) and/or fracture initiating member(s))) such that it does not result in the fragmentation of the second layer. For example, one or more (e.g. all) of the portions (e.g. fragments) of the second layer created in an impact may remain connected to one another (and to the remaining portion of the second layer). The second layer may be arranged to deform (e.g. bend) along the (at least partial) fracture(s) (e.g. at the point(s) and/or line(s) of weakness and/or the second layer at the protrusion(s) and/or fracture initiating member(s)). By decreasing the overall rigidity and enabling deformation of the second layer, the movement of the first layer and second layer with respect to each other may be facilitated.

In embodiments in which the second layer is curved, one or more of the segments defined by the point(s) and/or line(s) of weakness (and/or protrusion(s) and/or fracture initiating member(s)) are curved. The curvature of the segments may, for example, depend on the position of the segment within the second layer and the dimensions of the segment.

The segment(s) (defined by the point(s) and/or line(s) of weakness (and/or protrusion(s) and/or fracture initiating member(s))) may have any suitable and desired dimensions (e.g. size, shape). The dimensions (e.g. size, shape) of the segment(s) may be chosen depending on the intended application of the impact mitigating structure. For example, the segment(s) of a (pedal) bicycle helmet may have different (e.g. smaller) dimensions than a motorcycle helmet. The dimensions (e.g. size, shape) of the segment(s) may also depend on the size of the impact mitigating structure and/or the second layer.

When a plurality of segments are defined in the second layer, the segments of the second layer may be substantially identical to each other (e.g. in size or shape). For example, the segments may be substantially hexagonal (i.e. in the plane parallel to the surface of the second layer) and arranged to tessellate. For example, the segments may be substantially triangular (i.e. in the plane parallel to the surface of the second layer) and arranged to tessellate (e.g. interleave).

In some embodiments, the plurality of segments have a plurality of different sizes, shape and/or dimensions. For example, smaller segments may be arranged (e.g. located) in a region of higher surface curvature (of the second layer). In some (potentially overlapping) embodiments, smaller segments may be arranged (e.g. located) in a region of a perturbation (e.g. such as an indentation, vent, protrusion or uneven and/or irregular surface) on the first layer and/or the second layer.

Providing smaller segments in regions of higher curvature and/or perturbations helps to reduce the risk of geometric locking. Reducing the risk of geometric locking may dissipate energy more effectively from an impact and may increase the deflection of forces from an impact, reducing the amount of energy transferred through the helmet to the head being protected by the helmet.

In some embodiments, the second layer is arranged to fracture into plurality of separate segments. The separation of the segments from each other helps the segments to move independently from each other with respect to the first layer.

Preferably, the first layer and the second layer are designed to perform different functions in the impact mitigating structure. In a set of embodiments, the first layer comprises an impact (energy) absorbing layer. In at least preferred embodiments, such an impact mitigating structure is arranged to provide a degree of protection against bulk forces exerted in an impact. Thus preferably, the impact absorbing layer is arranged to absorb at least a portion of the normal component of the forces exerted on the impact mitigating structure during an impact. In a set of embodiments, the first layer has a thickness of between 5 mm and 50 mm, e.g. between 10 mm and 30 mm, e.g. approximately 25 mm.

The impact absorbing layer may be formed from any suitable and desired material, such as expanded polystyrene.

In one set of embodiments, the impact absorbing layer comprises a hollow cell structure, e.g. comprising a plurality of hexagonal cells (in cross section).

Preferably, at least some of the cells tessellate with each other. For example, the impact absorbing layer may comprise a micro-truss lattice or an out-of-plane honeycomb.

In some embodiments, the impact mitigating structure comprises a membrane. For example, the membrane may be provided (e.g. coated) on the first layer and/or between the first layer and the second layer. Alternatively, the membrane may be provided (e.g. coated) on another (e.g. additional) layer of the helmet. The membrane may provide a smooth surface to facilitate the movement of the second layer with respect to the first layer and/or the membrane. The membrane may provide a hard surface to support the protrusion(s) and/or fracture initiation member(s) acting against the second layer (to facilitate fracturing of the second layer). Thus the membrane is preferably between the second layer and the protrusion(s) and/or fracture initiation member(s). The membrane may have a thickness between 0.2 mm and 5 mm, e.g. between 1 mm and 2 mm.

In a set of embodiments, the second layer comprises a (e.g. resilient, hard, outer) shell. Preferably the second layer has a thickness that is (e.g. significantly) less than a thickness of the first layer. In a set of embodiments, the second layer has a thickness of between 0.05 mm and 5 mm, e.g. between 0.1 mm and 3 mm, e.g. between 0.5 mm and 2 mm, e.g. approximately 1 mm. The thickness of the second (e.g. outer) layer may be selected to be suited to the use of the helmet. For example, the second layer may be thicker for a motorcycle helmet than for a (pedal) cycling helmet.

Preferably the second (e.g. outer, shell) layer is formed from a rigid material. The second layer may be formed from a thermoplastic, e.g. polycarbonate or polymethyl methacrylate, or polyethylene terephthalate, or carbon fibre, or a ceramic material, or a paint, or a composite material; however, it could be made from any suitable and desired material. Preferable materials for forming the second layer (e.g. shell) have high strength to weight ratios. Preferable materials for forming the second layer (e.g. shell) have a notch impact toughness between 50 Jm⁻³ and 200 Jm⁻³.

In a set of embodiments the second layer comprises a solid film, forming the second layer or as a coating on the (e.g. outside of the) second layer. In one embodiment the solid film is formed from paint, e.g. a dry paint film. The film may be any suitable and desired thickness, preferably between 0.05 mm and 1 mm, e.g. between 0.1 mm and 0.5 mm.

Preferably the solid film is arranged to (at least partially) fracture, when the impact mitigating structure is subject to an impact. This may be thus be the second layer (at least partially) fracturing or the fracturing of the solid film may be arranged to initiate (at least partial) fracturing of the second layer.

Preferably, the adjacent surfaces of the first layer and the second layer (e.g. the surfaces arranged adjacent (e.g. to contact) one another), comprise low friction surfaces, e.g. having a large contact area (e.g. as a proportion of the whole surface area of the first and/or second layers). For example, the first layer and the second layer may comprise a low friction coating and/or be formed from a low friction (e.g. self-lubricating) material (e.g. Teflon), at least on their adjacent surfaces. Preferably the surfaces also have a large relative overlap, e.g. the second layer may extend over substantially the whole of the first layer. Low friction surfaces and large relative overlap may help the first layer and (at least a portion of) the second layer to move relative to each other, when subject to an impact.

The impact mitigating structure may comprise any suitable and desired number of layers. In some embodiments, the impact mitigating structure may comprise one or more further layers, in addition to the first and second layers. These further layer(s) may be arranged in any suitable and desired manner, and may have any suitable and desired composition. For example, a further (intermediate) layer may be positioned between the first (e.g. inner) layer and the second (e.g. outer) layer.

A further layer may comprise an impact absorbing layer, which may absorb forces exerted on a helmet during an impact, providing additional protection against head injuries and damage. In another potentially overlapping example, an additional layer may be positioned on (e.g. stacked on top of) the second layer. A further layer may comprise an outer shell, e.g. for aesthetic purposes. A further layer may comprise an inner padding layer, e.g. to provide fitting and comfort against a user's head. A further layer may comprise a membrane, as outlined above.

The first layer and the second layer, along with any further layers, may be connected to (mounted on) each other in any suitable and desired way. In one embodiment two or more of the layers are connected to each other mechanically. For example, the layers may be connected to each other by complimentary geometrical features of the layers or by one or more clips, latches, connectors, etc. In one embodiment the layers are connected to each other by an adhesive. For example, an adhesive that is hard and smooth (and remains so when at least one of the layers fractures) may help the layers to move easily relative to each other.

Preferably the connection of the first layer and the second layer, along with any further layers, is arranged to fix the positions of the first layer and the second layer (and any further layers) relative to each other. This helps to prevent slipping of the first layer and the second layer (along with any further layers) with respect to one another, which promotes (at least partial) fracturing of the second layer during an impact.

In a set of embodiments, the impact mitigating structure comprises a plurality of (secondary) layers each arranged to fracture in the manner of the second layer (e.g. such that the second layer comprises one of the secondary layers). Thus preferably one or more of a material property, a mechanical property and a geometrical property of the impact mitigating structure is arranged to facilitate, when the impact mitigating structure is subject to an impact, at least partial fracturing of one or more (e.g. each) of the secondary layers such that at least a portion of each of the secondary layers that at least partially fractures is able to move relative to the first layer, the second layer and/or another of the secondary layers. Providing a plurality of secondary layers that are arranged to fracture and move relative to other layer(s) of the impact mitigating structure helps to dissipate, absorb and deflect energy from an impact and/or help to prevent geometric locking that may occur throughout layers of a multi-layer impact mitigating structure.

In a set of embodiments, one or more (e.g. each) of the secondary layers comprises one or more points of weakness that are arranged to facilitate, when the impact mitigating structure is subject to an impact, at least partial fracturing of one or more (e.g. each) of the secondary layers, such that at least a portion of each of the secondary layers that at least partially fractures is able to move relative to the first layer, the second layer and/or another of the secondary layers.

In one embodiment the first layer, the second layer and/or one of the secondary layers comprises one or more protrusions (e.g. raised point(s) or line(s)) arranged to facilitate (e.g. initiate) at least partial fracturing of the secondary layer or the second layer when the impact mitigating structure is subject to an impact.

In one embodiment the impact mitigating structure comprises one or more fracture initiating members adjacent the secondary layer, wherein the one or more fracture initiating members are arranged to facilitate (e.g. initiate) at least partial fracturing of the secondary layer when the impact mitigating structure is subject to an impact.

Any suitable and desired number of the secondary layers may be arranged to fracture, as a result of an impact on the impact mitigating structure. The secondary layers may be arranged such that the impact force at which they fracture is similar to (e.g. the same as) the impact force at which the second layer is arranged to fracture. In such embodiments, when the force of the impact is at least the particular (e.g. threshold) force, the secondary layers are arranged to fracture.

In some embodiments, the force required to fracture a secondary layer (e.g. at one or more points of weakness) may be different to the force required to fracture the second layer. The (particular, e.g. threshold) force required during an impact to fracture each of the secondary layers may be different. Thus, for example, the impact mitigating structure may be arranged such that the number of the secondary layers that fracture depends on the (e.g. magnitude of the) impact force.

The secondary layers may comprise any (e.g. all) of the optional and preferred features outlined herein in relation to the second layer.

Thus, it will be appreciated that in embodiments in which the impact absorbing structure comprises an (e.g. first) inner layer and an (e.g. second) outer layer, that the (e.g. first) inner layer may not be the innermost layer of the impact mitigating structure or of the helmet. Similarly, the (e.g. second) outer layer may not be the outermost layer of the impact mitigating structure or of the helmet. Furthermore, the first and second layers may not be immediately adjacent (e.g. in contact with) each other. Simply, the (e.g. second) outer layer is provided outer of the (e.g. first) inner layer.

The impact mitigating structure may be any suitable and desired impact mitigating (e.g. absorbing) structure arranged to mitigate forces in (e.g. absorb energy from) an impact. While the above aspects and embodiments have been described primarily with respect to helmets, the Applicant has appreciated that the impact mitigating structure of the helmet may be applicable for other types of impact mitigating structures.

Thus when viewed from a further aspect the invention provides an impact mitigating structure comprising:

-   -   a first layer;     -   a second layer;     -   wherein one or more of a material property, a mechanical         property and a geometrical property of the impact mitigating         structure is arranged to facilitate, when the impact mitigating         structure is subject to an impact, at least partial fracturing         of the second layer such that at least a portion of the second         layer is able to move relative to the first layer.

It will be appreciated that the impact mitigating structure of this aspect of the invention may comprise any (e.g. all) of the optional and preferred features outlined herein with regard to any of the other aspects and embodiments of the invention.

According to a further embodiment of the helmet according to the invention, the second layer forms an outer shell of the helmet that is non-congruent with respect to the first shell, wherein, when the impact mitigating structure is subject to an impact, the outer shell is configured to fracture on impact such that at least a portion of the outer shell/second layer is able to move relative to the first layer. Thus, said fracturing facilitates relative movement of the outer shell (second layer) with respect to the first layer.

According to a further embodiment of the helmet according to the invention, the non-congruent outer shell, i.e. second layer, when the impact mitigating structure is subject to an impact, is configured to flatten during impact to facilitate relative movement of the outer shell with respect to the first layer.

Furthermore, according to an embodiment of the helmet according to the invention, the second layer is integrally formed with the first layer, the first layer forming an energy absorbing layer or a part of an energy absorbing layer.

According to a further embodiment of the helmet according to the invention, the impact mitigating structure comprises an intermediate layer being arranged between the first and the second layer configured to facilitate relative movement between the first and second layers.

Further, according to an embodiment of the helmet according to the invention, the intermediate layer comprises or is formed by a plurality of rolling elements. Preferably, in an embodiment, each rolling element of said plurality of rolling elements has a rolling resistance of less than 0.3. Furthermore, according to an embodiment, the rolling elements are hard and/or stiff rolling elements.

Further, according to an embodiment, each rolling element of said plurality of rolling elements is a spherical rolling element. Particularly, the respective rolling element can be a rigid sphere. Further, according to a preferred embodiment of the invention, each rolling element of said plurality of rolling elements comprises a diameter in the range from 1 mm to 4 mm.

According to yet another embodiment of the present invention, the impact mitigating structure comprises a fracturing mechanism that is configured to resist a relative movement between the outer second layer and the inner first layer. Particularly, in a preferred embodiment, the fracturing mechanism is configured to create a geometric locking or a mechanical locking between layers, for example between the first and the second layer. Particularly, in the context of the present invention, mechanical locking is understood to mean any mechanical interaction between two layers, particularly any type of interaction between layers in which an outer layer (e.g. second layer) collides with an inner layer (e.g. first layer) or other layers involved such that the desired movement of the outer layer with respect to the inner layer is impeded (and thus rendered less effective in terms of preventing injury to the person wearing the helmet).

Furthermore, according to an embodiment of the invention, the fracturing mechanism is configured to increase a resistance to rolling of the rolling elements.

Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 shows schematically a view of a conventional impact mitigating structure;

FIG. 2A shows schematically a cross-sectional view of a conventional helmet;

FIG. 2B shows schematically a cross-sectional view of the helmet of FIG. 2A as a result of an impact;

FIG. 3 shows schematically a view of a helmet in accordance with an embodiment of the present invention;

FIG. 4 shows schematically a view of a helmet in accordance with an embodiment of the present invention;

FIG. 5A shows schematically a view of an impact mitigating structure in accordance with an embodiment of the present invention;

FIG. 5B shows schematically a view of the impact mitigating structure of FIG. 5A after an impact;

FIG. 5C shows schematically a view of another impact mitigating structure of FIG. 5B after an impact;

FIG. 6 shows schematically a cross-sectional view of a helmet in accordance with an embodiment of the present invention;

FIG. 7 shows schematically a cross-sectional view of a helmet of FIG. 6 during an impact;

FIG. 8 shows schematically another cross-sectional view of a helmet of FIG. 6 during an impact;

FIG. 9 shows schematically a cross-sectional view of a helmet during an impact in accordance with an embodiment of the present invention;

FIG. 10 shows schematically a cross-sectional view of a helmet in accordance with an embodiment of the present invention;

FIG. 11 shows schematically a cross-sectional view of a helmet in accordance with an embodiment of the present invention;

FIG. 12 shows a schematically a cross-sectional view of a helmet in accordance with an embodiment of the present invention, wherein the helmet comprises a second layer in form of a non-congruent outer shell configured to fracture on impact; and

FIG. 13 shows the outer shell of FIG. 12 upon fracturing.

Impact mitigating structures act to protect a user or an object by absorbing and/or deflecting energy from an impact. In oblique impacts, which are a common form of impact, the impact mitigating structure may be subject to significant linear and tangential forces. These forces can cause a rapid deceleration of the user and/or object, which may cause serious damage. Embodiments of the present invention aim to provide an improved impact mitigating structure which reduces the risk of serious damage to the user being protected by the impact mitigating structure during an impact on the impact mitigating structure.

FIG. 1 shows schematically a plan view of a conventional impact mitigating structure 2. The impact mitigating structure 2 comprises a first layer 4 and a second layer 6 positioned on top of the first layer 4. The second layer 6 does not contain any points and/or lines of weakness, e.g. it is a uniform layer. The first layer 4 may formed from expanded polystyrene and the second layer 6 may be a polycarbonate shell. The impact mitigating structure 2 may be implemented in a helmet, e.g. in the helmet shown in FIG. 2A.

FIG. 2A shows schematically a cross-sectional view through a conventional helmet 20. The helmet comprises a first layer 24 and a second layer 26. For example, the first layer 24 may be an expanded polystyrene (EPS) foam impact absorbing layer and the second layer 26 may be an outer polycarbonate shell. The helmet 20 includes two vents 34, 35 which allows air to propagate through the helmet 20, providing an airflow for ventilation of the head (not shown) protected by the helmet 20. The vents 34, 35 are formed from openings in both the first layer 24 and the second layer 26.

FIG. 2B demonstrates the reaction of the helmet 20 shown in FIG. 2A to an impact. The force of the impact is represented in FIG. 2B by the arrow 38. As shown in FIG. 2B, the impact causes the second layer 26 to break at undefined locations in the second layer 26. During and/or after an impact, the sections created by the breaks in the second layer 26 may be obstructed and/or prevented from moving with respect to the first layer 24. This is due to the edges of the section of the second layer being caught in the vents 34, 35 (in the first layer). This is an example of geometric locking.

FIGS. 3 to 11 show impact mitigating structures according to various embodiments of the present invention.

FIG. 3 shows schematically a side view of a helmet 100 according an embodiment of the present invention. The helmet 100 is formed from an impact mitigating structure 102. The impact mitigating structure 102 has the following components: a first inner layer 104, a second outer layer 106 and a honeycomb layer 103. The second layer 106 is positioned outside of (e.g. stacked on top of) the first layer 104, and the first layer 104 is positioned on (e.g. stacked on top of) the honeycomb layer 103.

In the helmet 100, the first layer 104 is an impact absorbing structure comprising an EPS foam impact absorbing layer. The second layer 106 is a polycarbonate shell. The thickness of the second layer 106 is (substantially) less than the first layer 104. The first layer 104 and the second layer 106 comprises a plurality of vents 110, which allow air flow to the head protected by the helmet 100 (not shown).

The honeycomb layer 103 may provide additional impact absorption. The honeycomb layer 103 may also improve the fit of the helmet 100 to a user's head. The honeycomb layer 103 comprises a plurality of hollow cells. The hollow cells may allow an improved fit of the helmet 100 and/or improve the circulation of air throughout the helmet 100.

The second layer 106 includes multiple lines of weakness 108. The lines of weakness 108 may be formed by a series of indentations (e.g. notches), grooves, slots, perforations and/or impurities in the outer surface of the second layer 106.

The multiple lines of weakness 108 define the outline of a number of segments 112 of the second layer 106.

FIG. 4 shows schematically a side view of a helmet 200 according to another embodiment of the present invention. Similarly to the helmet 100 shown in FIG. 3 , the helmet 200 shown in FIG. 4 includes a first inner layer 204 (e.g. a EPS foam impact absorbing layer) and a second outer layer 206 (e.g. a polycarbonate shell).

The second layer 206 includes multiple lines of weakness 208, which define a number of segments 212. In FIG. 4 , the lines of weakness 208 track from the front to rear of the second layer 206, such that the segments 212 are formed as a series of elongated strips of the second layer 206. The width of the segments 212 may vary across the helmet. For example, the width of the segments 212 may be smaller towards the edges 216 of the second layer 206 (e.g. where the curvature of the second layer 206 is greater) and the width of the segments 212 may be larger towards the centre 214 of the second layer 206 (e.g. where the curvature of the second layer 206 is smaller).

FIG. 5A shows schematically a plan view of an impact mitigating structure 402 which may be incorporated into a helmet in accordance with an embodiment of the present invention. For example, the impact mitigating structure 402 may be implemented in the helmets 100, 200 shown in FIGS. 3 and 4 . Similarly to the impact mitigating structure 302 shown in FIG. 1 , the impact mitigating structure 402 shown in FIG. 5A includes a first layer 404 and a second layer 406 positioned on top of the first layer 404. However, unlike in FIG. 1 , the second layer 406 of the impact mitigating structure 402 comprises a series of lines of weakness 408. The lines of weakness 408 define the borders between different segments 412 of the second layer 406.

In the particular arrangement shown in FIG. 5A, the arrangement of the lines of weakness 408 forms (substantially) triangular interleaving segments 412.

The lines of weakness 408 are positioned more sparsely (i.e. having a greater separation between adjacent lines of weakness) towards the centre (of the surface) of the second layer 406. Therefore, larger segments 412 a are located at the centre of the second layer 406 and smaller segments 412 b are located towards the edges of the second layer 406.

The lines of weakness 408 shown in FIG. 5A are formed from perforations in the second layer 406. However, the lines of weakness may be may be perforations, apertures, grooves, slots, indentations (e.g. notches), voids and/or formed by impurities in the second layer 406.

FIG. 5B shows schematically a plan view of the impact mitigating structure 402 of FIG. 5A, which shows an example of the effect of an impact (exceeding a certain threshold) on the impact mitigating structure 402 shown in FIG. 5A. During and/or after the impact, the second layer 406 fractures along one or more (e.g. all) of the lines of weakness 408. In the particular example shown in FIG. 5B, all of the lines of weakness 408 have fractured. However, it will be appreciated that not all the lines of weakness 408 may fracture. The fracturing of the lines of weakness 408 may, for example, depend on the magnitude and the location of the impact (on the impact mitigating structure 402).

The fracturing of the lines of weakness 408 shown in FIG. 5B allows the segments 412 to (e.g. completely or partially) separate from each other. The segments 412 can then translate (e.g. move) relative to each other and the first layer 404. The fracturing of the lines of weakness 408 and movement of the segments 412 with respect to the first layer 404 dissipates a portion of the energy imparted by an impact so as to reduce the energy transferred to an object (e.g. a head) protected by the impact mitigating structure (e.g. the helmet). The effects of the fracturing of the lines of weakness 408 and the movement of the segments 412 will described in more detail in relation to FIGS. 6 to 9 .

FIG. 5C shows schematically a plan view of an impact mitigating structure 452, that shows another example of the effect of an impact (exceeding a certain threshold) on an impact mitigating structure 452 including a first layer 454 and a second layer 456. In FIG. 5C, the points or lines of weakness in the second layer 456 cause the second layer 456 to fragment (e.g. stochastically) into a number of irregular segments 462 when subject to an impact. The stochastic fragmenting of the second layer 456 may occur when fracturing occurs between (e.g. randomly) isolated points of weakness (e.g. that are not connected in a line).

FIG. 6 shows a cross-sectional view through a helmet 600 in accordance with an embodiment of the present invention. The helmet 600 comprises a first layer 604 and a second layer 606. The helmet 600 includes two vents 614, 615 which allow air to propagate through the helmet 500, providing ventilation to the head (not shown) protected by the helmet. Although not visible in FIG. 6 , the second layer 606 includes a number of points or lines of weakness defining a number of segments, e.g. as shown in FIGS. 3, 4, 5A or 5C.

Operation of embodiments of the present invention will now be described with reference to FIGS. 7 and 8 . FIGS. 7 and 8 show cross-sectional views of the helmet shown in FIG. 6 and demonstrate the possible behaviours of the helmet 600, in particular the behaviour of the second layer 606, as a result of an impact. It will be appreciated that the helmets of FIG. 3, 4 or 6 , or the impact mitigating structures shown in FIGS. 5A, 5B and 5C.

FIG. 7 shows a similar cross-sectional view of the helmet 600 as seen in FIG. 6 . The direction of the force of the impact is represented by the arrow 618. In an impact exceeding a certain threshold (e.g. a greater force than a user could reasonably exert on a helmet during normal use), sufficient energy required to at least partially fracture the lines of weakness (not shown) is imparted to the second layer 606. The fracturing of the lines of weakness fragments the second layer 606 into a plurality of disconnected, separated segments 612.

In the embodiment shown in FIG. 7 , the segments are (substantially) smaller than the dimensions of the vents 614, 615. This allows the segments 612 to move with respect to the first layer 604, for example by falling through the vents 614, 615, being ejected from the helmet or moving across of the outer surface of the first layer 604, without their continued movement being prevented by the vents 614, 615. The size of the segments 612 substantially reduces the risk of geometric locking (as described above with reference to FIG. 2B).

FIG. 8 shows an enlarged (zoomed in) cross-sectional view of the helmet 600 of FIG. 6 , demonstrating the behaviour of the segments 612 during and/or after an impact. The (surface of the) impacting object 620 is also shown in FIG. 8 . For example, the impacting object could be the surface of a road. The segments 612 formed in the impact (as described in relation to FIG. 7 ) may respond to the impact in various ways.

One or more of the segments 612 a may be ejected through the vents 614 in the first layer 604. Another set of segments 612 b may be ejected from the helmet, e.g. away from the outer surface of the first layer 604. The ejected segments 612 b may carry away a portion of the energy transferred to the helmet 600 from an impact, therefore dissipating energy from the impact. This reduces the energy transferred to the first layer 604 and then to the head (not shown) protected by the helmet 600.

Another set of segments 612 c may help to facilitate the movement of the impacting object 620 with respect to the first layer 604 (and thus the head protected by the helmet). The segments 612 c are configured to move (e.g. rotate, roll, translate) in order to facilitate the translation of the impacting object 620 with respect to the first layer 604. In such embodiments, it may be beneficial for the segments (i.e. the second layer) to be formed from a low friction material or be coated with a low friction coating. The movement of the segments 612 c may help to reduce the oblique forces transferred through the helmet 600, which helps to reduce the rotational movement of the head protected by the helmet during an impact.

FIG. 9 shows schematically a cross-sectional view through part of a helmet 700 during an impact with an impacting object 720. The helmet 700 seen in FIG. 9 is similar to the helmet shown in FIG. 6 , in that helmet 700 includes a first layer 704 and a second layer 706 which includes a plurality of lines of weakness (not visible) defining a plurality of segments.

However, in the embodiment shown in FIG. 9 , whilst the second layer partially fractures along the plurality of lines of weakness as a result of an impact, the second layer does not completely fracture such that a plurality of segments forming the second layer 706 complete separate from one another. Instead, as seen in FIG. 9 , the partial fracturing of the second layer along the lines of weakness allows the second layer 706 to deform (e.g. bend) along the lines of weakness. This increases the flexibility of the second layer 706, allowing to second layer 706 to bend and continue moving relative to the first layer 704, such that the second layer 706 moves into (e.g. the vents in) the first layer 704. This reduces the risk of the movement of the first layer 704 and the second layer 706 being prevented and/or obstructed (i.e. reducing the risk of geometric locking). The arrangement shown in FIG. 9 also reduces the risk of small segments of a fragmented second layer from causing damage, e.g. to the eyes of the user of the helmet.

FIG. 10 shows schematically a cross-sectional view of a helmet 800 in accordance with an embodiment of the invention. The helmet 800 includes a first, inner, impact absorbing layer 804 and a second, outer, shell layer 806. One or more protrusions 808 are formed on the first layer 804, facing the second layer 806. One or more protrusions 810 are formed on the second layer 806, facing the first layer 804. The protrusions 808, 810 may be point-like and act on the second layer 806 at a discrete point, or the protrusions 808, 810 may be longitudinally extended (in the form of a raised strip) to act on the second layer 806 along a line.

When the helmet 800 is subject to an impact, the protrusions 808, 810 act to concentrate the stress experienced by the second layer 806, as a result of the force of the impact, thus facilitating the fracturing of the second layer 806, e.g. at the locations at which the protrusions 808, 810 act on the second layer 806. As with previous embodiments, once the second layer 806 has been fractured, the fractured portion is then able to move relative to the first layer 804.

FIG. 11 shows schematically a cross-sectional view of a helmet 900 in accordance with an embodiment of the invention. The helmet 900 is similar to the helmet 800 shown in FIG. 10 , in that it includes a first, inner, impact absorbing layer 904 and a second, outer, shell layer 906. However, instead of protrusions 808, 810 formed on the first and second layers 804, 806, the helmet 900 comprises a plurality of fracture initiating members 908 (in the form of hard balls or strips) positioned between the first layer 904 and the second layer 906. The fracture initiating members 908 may be point-like balls and act on the second layer 906 at a discrete point, or the fracture initiating members 908 may be longitudinally extended (in the form of a strip) to act on the second layer 906 along a line.

The helmet 900 also comprises a hard membrane coating 910 on the first layer 904, between the first layer 904 and the fracture initiating members 908.

When the helmet 900 is subject to an impact, the fracture initiating members 908 act to concentrate the stress experienced by the second layer 806, as a result of the force of the impact. The hard membrane coating 910 prevents the fracture initiating members 908 from becoming embedded in the first layer 904, thus facilitating the fracturing of the second layer 906, e.g. at the locations at which the fracture initiating members 908act on the second layer 806. As with previous embodiments, once the second layer 906 has been fractured, the fractured portion is then able to move relative to the first layer 904.

FIGS. 12 to 13 show schematic cross-sectional views of a helmet 1000 according to an embodiment of the present invention. The helmet 1000 comprises an impact mitigating structure 1002. In particular, the impact mitigating structure 1002 has the following components: a first inner layer 1004, a second outer layer 1006 and an intermediate layer 1005. Particularly, the second layer 1006 is positioned outside of (e.g. stacked on top of) the first layer 1004, and the intermediate layer 1005 is arranged between the second and the first layer 1006, 1004. The first layer 1004 can comprise an energy absorbing layer. Further, according to an embodiment, the intermediate layer 1006 can comprise or be formed out of a plurality of rolling elements 1007 that can be designed as described herein (e.g. rigid spheres having e.g. a diameter in the range from 1 mm to 4 mm). The rolling elements 1007 facilitate relative movement of the second layer 1006 with respect to the first layer 1004 by allowing movement of the second layer 1006 (or parts thereof) with the rolling elements 1007 rolling underneath the second layer 1006 (or parts thereof) upon an impact. However, also intermediate layers 1005 are conceivable that do not comprise such rolling elements 1007, but facilitate relative movement due to alternative material properties or structures.

Particularly, as shown in FIGS. 12 and 13 , the second layer 1006 forms a non-congruent outer shell 1006 with respect to the underlying first layer 1004. FIG. 12 shows the outer shell 1006 before an impact exerting a force F onto the helmet 1000.

As indicated in FIG. 13 , the non-congruent outer shell 1006 is configured to fracture on impact into a plurality of fractured portions 1060 that then move relative to the inner first layer 1004 on the intermediate layer 1005, wherein (if present) the rolling elements 1007 facilitate the movement of the individual fractures portion 1060 with respect to the first layer 1004 by rolling underneath them. In other words, the non-congruent outer shell 1006 breaks upon impact to facilitate sliding or moving of the fractured portions 1060 on the rolling rolling elements 1007 (in case the intermediate layer 1005 comprises rolling elements 1007) or on an alternative intermediate layer without rolling elements 1007.

Thus it will be appreciated by those skilled in the art that an impact mitigating structure according to embodiments of the present invention, in which the one or more points of weakness are arranged to fracture as a result of an impact to facilitate the movement of a first layer and a second layer with respect to the each other, helps to reduce the forces transferred through the impact mitigating structure, e.g. to a user or object being protected by the structure. This may provide benefits over known impact mitigating structures and, particularly when the impact mitigating structure is a helmet, provide significant benefits over known helmets, e.g. in helping to reduce brain injuries. It will further be appreciated however that many variations of the specific arrangements described herein are possible within the scope of the invention, such as combinations of features taken from the embodiments shown. 

1. A helmet comprising an impact mitigating structure, the impact mitigating structure comprising: a first layer; and a second layer; wherein one or more of a material property, a mechanical property and a geometrical property of the impact mitigating structure is arranged to, when the impact mitigating structure is subject to an impact, facilitate at least partial fracturing of the second layer such that at least a portion of the second layer is able to move relative to the first layer.
 2. The helmet as claimed in claim 1, wherein the impact mitigating structure is arranged to set a particular threshold force of the impact at or above which the second layer is arranged to fracture.
 3. The helmet as claimed in claim 1 or 2, wherein the particular force at which the second layer is arranged to fracture is between 10 N and 100 N, e.g. between 30 N and 70 N, e.g. approximately 50 N.
 4. The helmet as claimed in claim 1, 2 or 3, wherein the second layer has a fracture toughness of between 0.1 MPa m^(1/2) and 10 MPa m^(1/2), e.g. between 0.5 MPa m^(1/2) and 5 MPa m^(1/2), e.g. between 1 MPa m^(1/2) and 3 MPa m^(1/2).
 5. The helmet as claimed in any one of the preceding claims, wherein the first layer and/or the second layer comprises one or more protrusions arranged to facilitate at least partial fracturing of the second layer when the impact mitigating structure is subject to an impact.
 6. The helmet as claimed in any one of the preceding claims, wherein the impact mitigating structure comprises one or more fracture initiating members adjacent the second layer, wherein the one or more fracture initiating members are arranged to facilitate at least partial fracturing of the second layer when the impact mitigating structure is subject to an impact.
 7. The helmet as claimed in any one of the preceding claims, wherein the second layer is shaped to form one or more points and/or lines of weakness in the second layer, wherein the one or more points and/or lines of weakness are arranged to facilitate at least partial fracturing of the second layer.
 8. The helmet as claimed in claim 7, wherein the second layer comprises a plurality of points and/or lines of weakness and the second layer is arranged to at least partially fracture at least one of the plurality of points and/or lines of weakness or between at least two of the plurality of points and/or lines of weakness.
 9. The helmet as claimed in claim 7 or 8, wherein the one or more points and/or lines of weakness are defined by material properties of the second layer.
 10. The helmet as claimed in claim 7, 8 or 9, wherein the second layer comprises a material having one or more impurities therein, wherein the one or more impurities defines the one or more points and/or lines of weakness.
 11. The helmet as claimed in any one of claims 7 to 10, wherein the second layer comprises one or more fibres and/or one or more seeding particles, wherein the one or more fibres and/or the one or more seeding particles are arranged to form the one or more points and/or lines of weakness.
 12. The helmet as claimed in any one of claims 7 to 11, wherein the one or more points and/or lines of weakness are defined by geometrical properties of the second layer.
 13. The helmet as claimed in any one of claims 7 to 12, wherein the thickness of the second layer at the one or more points and/or lines of weakness is less than the thickness of the surrounding regions of the second layer.
 14. The helmet as claimed in any one of claims 7 to 13, wherein the one or more points and/or lines of weakness comprise one or more indentations, voids, grooves, slots and/or apertures in the second layer.
 15. The helmet as claimed in any one of the preceding claims, wherein the first layer and/or the second layer comprises one or more protrusions and/or the impact mitigating structure comprises one or more fracture initiating members adjacent the second layer and/or the second layer comprises one or more points and/or lines of weakness, wherein the one or more protrusions, the one or more fracture initiating members and/or the one or more points and/or lines of weakness are arranged to define one or more segments of the second layer.
 16. The helmet as claimed in claim 15, wherein the second layer comprises between 3 and 1000 segments, e.g. between 50 and 500 segments, e.g. between and 300 segments, e.g. between 100 and 150 segments.
 17. The helmet as claimed in claim 15 or 16, wherein the segments extend over the entirety of the second layer.
 18. The helmet as claimed in claim 15, 16 or 17, wherein the segments are arranged relative to the geometrical features of the helmet.
 19. The helmet as claimed in any one of claims 15 to 18, wherein the segments are arranged to surround one or more vents in the helmet.
 20. The helmet as claimed in any one of claims 15 to 19, wherein the second layer is arranged to fracture, when the impact mitigating structure is subject to an impact, to facilitate at least partial detachment of at one or more segments from the second layer.
 21. The helmet as claimed in claim 20, wherein the one or more at least partially detached segments are arranged to, when the impact mitigating structure is subject to an impact from an object, facilitate movement of the second layer with respect to the impacting object.
 22. The helmet as claimed in claim 20 or 21, wherein the one or more at least partially detached segments are arranged to, when the impact mitigating structure is subject to an impact, be freed from the impact mitigating structure.
 23. The helmet as claimed in claim 20, 21 or 22, wherein the second layer is arranged to, when the impact mitigating structure is subject to an impact, bend between the partially detached segment and the second layer.
 24. The helmet as claimed in any one of claims 15 to 23, wherein the second layer comprises a plurality of smaller segments arranged in a region of higher surface curvature of the second layer and/or in a region of a perturbation on the first layer and/or the second layer.
 25. The helmet as claimed in any one of the preceding claims, wherein the first layer comprises a hard membrane between the first layer and the second layer.
 26. The helmet as claimed according to one of the preceding claims, wherein the second layer forms an outer shell that is non-congruent with respect to the first layer, wherein when the impact mitigating structure is subject to an impact, the outer shell is configured to fracture such that at least a portion of the outer shell is able to move relative to the first layer.
 27. The helmet as claimed in claim 26, wherein the outer shell, when the impact mitigating structure is subject to an impact, is configured to flatten to facilitate relative movement of the outer shell with respect to the first layer.
 28. The helmet as claimed in claim 1 or as claimed in one of the claims 2 to 27, wherein the second layer is integrally formed with the first layer, the first layer forming an energy absorbing layer or a part of an energy absorbing layer.
 29. The helmet as claimed in one of the preceding claims, wherein the impact mitigating structure comprises an intermediate layer configured to facilitate relative movement between the first and second layers.
 30. The helmet as claimed in claim 29, wherein in the intermediate layer comprises a plurality of rolling elements.
 31. The helmet as claimed in claim 30, wherein each rolling element of said plurality of rolling elements has a rolling resistance less than 0.3.
 32. The helmet as claimed in claim 30 or 31, wherein the rolling elements are hard and stiff.
 33. The helmet as claimed in one of the claims 30 to 32, wherein each rolling element of said plurality of rolling elements is spherical.
 34. The helmet as claimed in one of the claims 30 to 33, wherein each rolling element of said plurality of rolling elements comprises a diameter in the range from 1 mm to 4 mm.
 35. The helmet as claimed in one of the preceding claims, wherein the impact mitigating structure comprises a fracturing mechanism that is configured to resist a relative movement between the second layer and the first layer.
 36. The helmet as claimed in claim 35, wherein the fracturing mechanism is configured to create a geometric locking or a mechanical locking between layers.
 37. The helmet as in claim 35 or 36, wherein the fracturing mechanism is configured to increase a resistance of rolling of the rolling elements. 